The digital watermark is a technology for embedding additional information into image data and audio data so that the additional information is not recognizable by humans. When copyright information of content or purchase information of a user is embedded as digital watermark, if the content is fraudulently leaked, the presence/absence of copyright information and the source of the leak can be identified.
For example, a digital-watermark embedding device embeds digital-watermark information to content in conjunction with a unique secret data flag, and the digital-watermark information is extracted through detection of the secret data flag. The secret data flag is also called a “synchronization flag”. For example, Japanese Laid-open Patent Publication Nos. 2005-109918 and 2001-202443 disclose technologies in which a secret data flag is generated from a random number and the generated secret data flag is embedded into content.